1. Field of the Invention
The present invention relates to a hydraulic control apparatus for a hydraulic lock-up clutch assembled within a torque converter disposed between a prime mover of an automotive vehicle and a power transmission.
2. Description of the Prior Art
In Japanese Patent Laid-open Publication No. 62(1982)-72960, there has been proposed a hydraulic control apparatus for a lock-up clutch of the direct-coupling type which includes a lock-up control valve switched over under control of a solenoid valve for selectively supplying fluid under pressure into an engagement-side fluid passage or a release-side fluid passage of the lock-up clutch. When supplied with fluid under high pressure at its engagement-side fluid passage under control of the control valve, the lock-up clutch is engaged to establish a direct connection between input and output elements of the torque converter. When supplied with fluid under high pressure at its release-side fluid passage under control of the control valve, the lock-up clutch is disengaged to release the direct connection between the input and output elements of the torque converter.
In a disengaged condition of the lock-up clutch, the release-side fluid passage of the lock-up clutch is communicated with a fluid supply passage of the lock-up control valve to permit fluid under pressure flowing therefrom into the release-side fluid passage, while the engagement-side fluid passage of the lock-up clutch is communicated with an inlet-side fluid passage of a cooler to permit fluid under pressure flowing into the cooler from the lock-up clutch therethrough. When the lock-up clutch is engaged, the engagement-side fluid passage of the lock-up clutch is communicated with the fluid supply passage of the control valve to permit fluid under pressure supplied into the lockup clutch from the fluid supply passage therethrough, while the release-side fluid passage of the lock-up clutch is communicated with a drain passage to discharge therethrough the fluid under pressure from the lock-up clutch.
In the hydraulic control apparatus, however, if the fluid under pressure may not be smoothly discharged through the drain passage, engagement of the lock-up clutch will be delayed or suddenly effected. To avoid such a problem, it is required to provide a fluid passage of large diameter as the drain passage for effecting smooth discharge of the fluid under pressure in the case that such a fluid passage of large diameter is formed as the drain passage in a housing structure of the control valve, the arrangement of the drain passage in the housing structure becomes complicated in construction. This results in an increase of the manufacturing cost of the hydraulic control apparatus.